Let's continue working on the East German (DDR/GDR) Robotron KC 85 "small computer" that runs on the East German Z80 clone called the U880 (UB880 in this case).

A lot of excellent information ... and photos ... can be found here:
https://floooh.github.io/virtualkc/p0...

After fully disassembling (with some effort) in Part 2, it was possible to make some small repairs. There were a few 100nF ceramic capacitors in poor state, and one that had been removed completely. I also further removed the RF video output cabling and fixed a bridge track on the expansion board.

I cabled up an RGBS cable for the GBS8200 according to the documentation, using the FBAS (composite video?) signal for the synchronisation input.

The Brow is the topside of the edge connector, and the Arow is the underside (which is pretty standard I guess). Pins are counted from the right to left.

Blue = B4
Green = B6
Red = B8
Synch = A10 (FBAS)
Ground = B3 (or B6, B7, B9)

And ... it seems to be working! It booted into HCCAOS 3.2, where CAOS stands for "Cassette Assisted Operating System". The latest available version is 4.2 but I don't know if that's for the KC 85/4 only and I need to stick to the 3.x branch for the KC 85/3.

Now that I've figured out the RGBS pinout, I'll probably wire up a 5pin socket (or 6+ pin to include audio?) and install that on the case. I want to also replace the Tesla MA7812 linear regulator in the power supply with a modern version from EZSBC.COM to reduce on waste heat and power draw.

If possible to upgrade the CAOS version to the latest, I'll give that a go ... will need to desolder the ROM.

The most challenging part will likely be the reassembly of the machine & case.